Measurement of stream cross section using ground penetration radar with Hilbert–Huang transform Yen-Chang Chen,* Su-Pai Kao and Jhong-Hua Wu Department of Civil Engineering, National Taipei University of Technology, Taipei, Taiwan Abstract: This study presents a new method to measure stream cross section without having contact with water. Compared with conventional measurement methods which apply instruments such as sounding weight, ground penetration radar (GPR), used in this study, is a non-contact measurement method. This non-contact measurement method can reduce the risk to hydrologists when they are conducting measurements, particularly in high flow period. However, the original signals obtained by using GPR are very complex, different from studies in the past where the measured data were mostly interpreted by experts with special skill or knowledge of GPR so that the results obtained were less objective. This study employs Hilbert–Huang transform (HHT) to process GPR signals which are difficult to interpret by hydrologists. HHT is a newly developed signal processing method that can not only process the nonlinear and non-stationary complex signals, but also maintain the physical significance of the signal itself. Using GPR with HHT, this study establishes a non-contact stream cross-section measurement method with the ability to measure stream cross-sectional areas precisely and quickly. Also, in comparison with the conventional method, no significant difference in results is found to exist between the two methods, but the new method can considerably reduce risk, measurement time, and manpower. It is proven that the non-contact method combining GPR with HHT is applicable to quickly and accurately measure stream cross section. Copyright © 2013 John Wiley & Sons, Ltd. KEY WORDS cross section; ground penetrating radar; Hilbert–Huang transform; hydrological measurement; stream Received 23 April 2012; Accepted 7 February 2013 INTRODUCTION Stream cross section is the combined result of all climatological and geographical factors. Measurement of stream cross section is always the basic task of a hydrologist. Through continuous measurements of stream cross section, water stage, and discharge, the obtained hydrological data can be applied for hydraulic structure design, flood control projects, water resources planning, and ecological restoration. However, current climate change situation leads to a wide gap in streamflow between high flow period and low flow period, and the stream cross section changes drastically. Therefore, it is necessary to increase the frequency and precision in measurement of stream cross section. Conventional ways of measuring stream cross section, such as sounding weight method, are contact measurement methods, which have higher risk and usually entail enormous expense, labor, and time. Consequently, hydrologists in recent years have been actively looking for a more economical, safer, and highly efficient non-contact method to measure stream cross section. The methods of discharge measurement and cross-section measurement were first developed between the 15 th and 16 th century. During this period, Benedetto Castelli and other scholars, such as Leonardo da Vinci, established measure- ment techniques and an initial discharge theory by using the product of flow velocity and wetted cross section to estimate discharge (Frazier, 1974). The stream cross section is often measured with sounding weights; such technique has long been used and remained being utilized even today. Manual methods are generally used in situations where use of electronic echo sounding systems are not practical or accurate. For example, a stream with dense bottom vegetation, irregular jetty stone, or shallow water may give false signals electronically. In streams, the sounding weight is popular to measure cross section. A sounding weight is lowered from a bridge or a boat at the desired location. By measuring the length of cable, the depth of water is obtained. As shown in Figure 1, the sounding weight drifts downstream in swift water and the depths as measured by the conventional methods will be in error, being too large. The correction for the error has two parts, the airline correlation and the wetline correlation. These correlations depend on vertical angle and total horizontal drag on the sounding line (Rantz, 1982). However, the vertical angle and the assumptions to correct *Correspondence to: Yen-Chang Chen, Associate Professor, Department of Civil Engineering, National Taipei University of Technology, Taipei, Taiwan. E-mail: yenchen@ntut.edu.tw HYDROLOGICAL PROCESSES Hydrol. Process. 28, 2468–2477 (2014) Published online 18 April 2013 in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/hyp.9755 Copyright © 2013 John Wiley & Sons, Ltd.